The aspect or'time lag in long -term cosmic ray intensity variation with solar activity *

Transcription

1 Indian Journal of Radio & Space Physics Vol. 28 October 1999 pp The aspect or'time lag in long -term cosmic ray intensity variation with solar activity * Mahendra Singh Department of PhysicsGoVLT.R.S.Coli ege Rewa (M P) and S K Nigam Department of Physics A.p.s.University Rewa (MP) and Pankaj K Shri vastava Department of Physics Govt New Science College Rewa (MP) Received 5 April 1999; revised received 22 July 1999 The relationship between the long-term vari ation of the cosmic ray intensity and solar acti vity considering the different time lags has been investigated during the period Several aspects of the long-term variati ons of cosmic ray intensity have been studied adopting the K-se ri es analysis method during the ascending and descending phases of four successive sunspot solar cycles (i.e. from 19 to 22). Correl at ive analysis is the basis and has been applied for several time intervals. On the overall correlative analysis it has been observed that th e second parts of ascending phase for sunspot cycles are more effective in producing the cosmic ray modulation. The accumulative results of these effects produce long-term trends in cosmic ray intensity. 1 Introduction Generally sunspot numbers are used as one of the reliable and easily available solar parameters to measure solar activity. Unfortunately no unique measure of solar activity which can be used as solar parameter in cosmic ray studies is appropriate. The sunspot numbers as an active and reliable parameter are produced mainly because so lar fl ares emanate from sunspot regions. However the index is a quantified value which is counted as 10 and each spot is counted as I weighted for the penumbral and umbral structure. Therefore one can say that the sunspot area is a better measure of the active region of the solar disk. It is well known that cosmic ray intensity variation shows inverse correlation with sunspot number for I 1/22 years. But generally it seems that the maximum 1 minimum of SUl1sPOt numbers does not coincide with minim um Imaxi mum of cosmic ray inten sity. A time lag of 2-10 months arc general ly found wh ich also shows another aspect in long-term cosmic * The paper was prcsentcd at the 10'h ationd l SIace Science Symposium held at Physical RCSCilich Laboratory Ahmedabad. during Nov. 1')')7 ray modulation. Recently Popielawska I and others 23 have reported a detailed study considering a lag between cosmic ray intensity data and sunspot numbers to show the correlation between cosmic rays and sunspot numbers fo r all phases of success ive sunspot cycles. For further verification of the results a detailed study has been made in this paper to derive the best correlation coefficient after dividing each sunspot cycles in four different phases. 2 Data and method of analysis The pressure corrected monthly mean val ues of cosmic rays are obtai ned from the data of Climax (2.9 GeV ) and Huancayo (12.9 GeV ) neutron monitors for the period of The mean va lues of sunspot numbers have been taken from the Solar Geophysical Report as a reliable so lar parameter. To derive the aspect of time lag in longterm cosmic ray inte nsity the techniques of K- series have been adopted. The K-series is a modified version of moving 1 sl ide average techniques wh ich shows long-term pro fil e of data set. In thi s v:ay on e can exclude the short-term fluctuati oli s of noise in clata set. This technique wa:; adopted for the first time by Shea and

2 212 INDIAN J RADIO & SPACE PHYS OCTOBER 1999 Smart 4 in 1985 for deriving the long-term profile of cosmic ray intensity and its relationship with sunspot numbers.in thi s method the average value of any X data series (monthly/yearly/daily) for Kth month has been correlated against the two-month average of Y series (Kl series) for K and K-I and-similarly for the 3-month Y average (K2 series) of months K K-I and K-2 etc. up to n months lag (K-n i. e. up to the Kn series). From K- series analysis one can obtain the best correlation coefficient disregarding the lag interval. In this study a correlative analysis has been done considering zero month lag to 19-month lags. First monthly mean values of cosmic rays (for a certain period) are correlated with monthly mean values of sunspot numbers and is said to be zero lag. Further two months moving average has been taken for sunspot numbers and correlated with month Iy mean values of cosmic rays. It is said to be one month lag analysis. Similarly 3 to 19 months moving average and derived correlation coefficients up to 19 months lag have been found out. 3 Results and discussion To observe the relationship between cosmic ray intensity and sunspot numbers for the four successive solar cycles (19-22) the correlation coefficient between the monthly mean values of these two parameters has been derived. The pressure corrected monthly mean cosmic ray values of two neutron monitors namely Cl imax (2.99 GeV) and Huancayo( 12.9 GeV}-one of low and the other of high rigidity are taken into consideration. [t has been observed since last 4-5 decades that the long-term cosmic ray intensity are generally anti-correlated with solar activity5-7. It has also been noticed that the exact month of solar activity maximum/minimum does not coincide with cosmic rays minimum/maximum. Such a difference in monthly/yearly mean values of sunspot numbers with cosmic rays monthly/yearly mean values is known as time lag. The average mean values of sunspot numbers R z have been taken as a solar activity parameter to derive the correlation coefficient considering zero time lag Studies on long-term modulation of galactic cosmic rays require an extended set of homogeneous data free of terrestrial induced effect. Good quantitative results could be derived from the data of neutron monitors. Therefore to get better results the Climax and Huancayo neutron monitoring stations have been selected for the present study..figures I and 2 illustrate the yearly mean values of sunspot numbers along with cosmic ray intensities for the period which cover the sunspot solar cycles Figuresl and 2 show the II -yr modul ati on cycle for Climax and Huancayo neutron monitors respectively. Ascending and descending phases of sunspot cycles are well noticed in these I I-yr modulation cycles. One can clearly observe the anti-correlation of sunspot numbers (R z ) with cosmic ray intehsity (CRI) from Figs I and 2 and also some deviation (time lag) in cosmic ray maxima with sunspot minima for all the sunspot solar cycles. In this study all the sunspot cycles have been divided in ascending and descending phases. All these ascending and descending phases are again divided into two parts as part I and part 2 according to the sunspot numbers and duration of half the period from the minimum to next maximum. Figure 3 shows the results of correlation for all phases. A good anti-correlation is observed between R z and cosmic ray intensity at neutron monitor energies.it is always found that r > Correlation coefficients for that second part of ascending phases are generally found to be higher than for another intervals. Correlation coefficient for As 2 I is found to be lower than those as are for the other phases. Earlier works 1-3 reported high anticorrelation between Rz and cosmic rays (correlation coefficient r > - Q.9) for all the phases of solar cycles except for the declining (minimum period) phase of sunspot cycles and 22. Results obtained in the present analysis is in agreement with the earlier works l - 3. These declining and minimum solar activity periods are dominated by high speed solar wind streams coming from coronal holes which affect the cosmic ray propagation in interplanetary medium. To derive the new aspects in long-term cosmic ray intensity and to look into the problem in more detail the correlation of cosmic ray intensity vs R z have been repeated considering a lag between sunspot activity and cosmic ray intensity. The same pressure corrected monthly mean cosmic ray values have been taken from the two neutron mon itors C I i max and Huancayo. The K- series time lag correlative analysis has been done using the method described by Shea and Smart 4. [n this process the monthly cosmic ray intensity for the Kth month has been correlated against the two months average of Rz (Rz I series) for mqnths K and K- I and similarly for the three months average of Rz (Rz 2 series) for months K K-I and K-2 etc. up to 19 months lag up to Rz 19 series. By adopting this method the best correlation coefficient

3 SINGH 1:1 al. : TIME LAG IN LONG-TERM COSMIC RAY INlEN6ITY VARIATION : ' --CRI(CLIMAX) R z '100 I 'J CRI Rz ' ' SOLAR CYCLE-" SOlAR CYCL1: ' CYCLE ' r----~------r ~------r_----_----~----~~ ~ ~5 ' YEAR I Fig. I-Yearly mean values of sunspot numbers (R z ) and cosmic ray intensity (-C R I) data of Climax station for the period (The period of extent of so lar cycles are divided by vertical lines) u CRI (HUANCA YO) 1 flo R z " CR j Rz SOLAR CYCle. 19 SOLAR CYCLE '- ~.. SOLAR CYClE 21 ~ '. SOLM eyel ~----~---~--~ ~ ~ 0 Igso ~ 2UOO YE.AR Fig. 2-Same as Fig. I but for Huancayo neutron monitor station 40

4 214 INDIAN J RADIO & SPACE PHYS. OCTOBER 1999 f- Z w [) t:i: u.. w 00 u «Z.J 00-0:: I-w «N ~ o u 0 0 J' " 0 6. f (0"0+oo91TIO'.S+0'08 1!~ ~:.:; " ~ I IiI i-i! I h!! 0 1 tt! t. I!! I! I I!! D I U I II I II 1 U I U I U I U As Ai 0 As AS SUNSPOT CYCLES PHASE Fig. 3-Values of correlation coefficients between R z and cosmic rays at zero ti me lag fo r the ascend ing phase I and II. and descending p~e J and II coveri ng the fo ur successive solar cycles fro m 19 to Z2 (Error bars have been shown fo r each points. Circles and rectaogles are representing the values for Huancayo and Climax neutrons). and their respective time lag (in month) have been derived. Analysis of the above work shows signi ficant correlation between R z and ne utron-monitor-cosmic ray intensity fo r all the ascending and descendi ng phases of solar cycles Figure 4 shows the further values of correlation coefficient between the cosmic rays and sunspot numbers considering the ti me lag up to 19 months. It is qui te apparent fro m Fig. 4 that the correlation coeffi cients are higher ( r > - 0.9) for all the ascendi ng and descending phases of fo ur successive su nspot solar cycles Some low coefficients are noted during the As 1 phases of solar cycles and DI phase of solar cycle 2 I. However the new result comes from the analysis of the lag changes associated with the best correlation coefficients. Figures 5 and 6 show for cach phase of sunspot cycles thc number of months to be averaged in the Rz data set to obtain the best values. More importantly is shown the end poir{t of the time interval considered to perform the R z I series ( i = end point in month ) related to the best correlation coefficient. It is noted that the va lues are obtained wi th several subsequent R z I series. A new aspect has been noted to observe the lag interval values from the bar diagrams of Figs 5 and 6. For ascending phases the best correlation coefficient is obtained with th e R z series derived by averagi ng R z monthly va kjes fo r more than 15 months. On the other hand it is found to be less than 7 months for descending phases except for solar cycle 22. The second part of solar cycle 21 (21 DIl) a lso shows best correlation for more than 12 months. I- :z III 1] i:i: 0-0 ()'2 Lo ~ ~ O '~ OW -~ 1-1= ~i= CH ~- 8~ 0'8 I- 12 c:c 1-0 IU I U I U I Il I UIIi As AS 0 A As SUNSPOT CYCLES PHASE Fig. 4-Same as Fig. 3. but for best correlation coeffic ient considering the time lag between R and cosmic rays.::: E 0 E r.:5 <...J W ~ 1= I~ 12 'I 0 HUANCAYO - 0 CLIMAX - ~ I I n ~ 19 Ai l 19Ai ll " 1 As II 21 "( 21AS I1 22 Ml 22AsU 1 SUNSPOT CYCLES ASCENDING PHASE Fig. 5--l-listograms showing the time lag in months. whi ch gi ve best correlation coefficients between R z and cosmi c rays for th e ascending phases of sunspot cycles & HUANCAYO O CLI M AX - --ell SUNSPOT CYCLES DESCENDING PHASE Fig. 6--Samc as Fig. 5. but for descend ing phases of sunspot eycles t ' t

5 SINGH el al.: TIME LAG IN LONG-TERM COSMIC RAY INTENSITY VARIATION 215 The results of time-varying rigidity-dependent effects on time lag analysis are presented here for al l the four sunspot solar cycles using the data from Climax/Huancayo pair of neutron monitor stations (geomagnetic cut-off rigidities:2.9 GeV 112.9GeV) for the period i 996. Figures 3 and 4 show higher correlation coefficients for Climax neutrons than the coefficients obtained from Huancayo neutrons. High correlation between R z and CRI is also observed for the high rigidity particles which indicates rigidity dependence in time lag for long-term cosmic ray modulation. There are various explanations for rigidity dependent phase lag that leads to the hysteresis effect also in the modulation of cosmic rays for a full solar cycle. Rigidity dependence in cosmic rays follows quite naturally from the higher mobility of high energy particles relative to low energy ones leading to faster results of modulation effects of high energy particles. It is noted from the analysis that the best correlation is observed fo r all the ascending phases of sunspot cycles except 21 As!' Lag intervals associated with Climax data are fou nd different from those related to Huancayo data while they are found to be the same for some cases such as ascending sunspot cycles. The cosm ic ray modulation depends upon various factors namely the magnitude and direction of regular magnetic fi elds.the level of magnetic disturbances the solar wind speed the size and shape of heliomagnetosphcrc. Becaus the overall sign of the interplanetary magnetic fie ld changes from one sunspot minimum to the next. the drift motion changes high and the occurrence of the cosmic rays is quite different. In the quiet hcliosphcrethe cosmic. rays propagate inward by a combination of random walk and guiding center drifts which are coherent over large distances. To investigate the long-term behaviour of the cosmic ray intensity one of the solar features (Green Corona intensity) has been studied foi1 different phases of sunspot cycles 4. The Green Corona features (spectral lines FeX IV 530 3mm) indicate high brightness during the second part of almost all the ascending phases of sunspot cycles. There are several intense solar features in interplanetary med ium duri ng second part of each ascend ing phase of sunspot cycles for producing discrepancies in cosmic ray behaviour. The response of cosmic ray particle to solar activity during ascending phases of solar cycles is different from that related to descending phases. It is inferred that this behaviour anses from a di ss imilar heliospheric condition during successive solar activity cycles. As we know that the heliomagnetic cycle lasts about 22 years due to the reversing action of the sun 's magnetic fi eld around sunspot maxima there is an opposite sign of magnetic dipole moment during consecutive hel iomagnetic cycles 8. Hence charged particles in interplanetary medium may suffer the effects fro m gradient and curvature of 3 D-magnetic fiel d. Acknowledgement The authors are thankful to the World Data Center A for providing the data for the present study. References I Popielawska B Planet & Space Sci (U "j AO( 1992)81 I. 2 Shrivastava P K Proceedings of 25th Illternational COllference on Cosmic Rays Durban South Africa Vol. 2; 1997 p65. 3 Singh M Nigam S K & Shrivastava P K Proc Nail Acad Sci India A Phys Sci (India) 68 (1998) III. 4 Shea M A & Smart D L Proceedings oj I Yth Intemallonal Conference on Cosmic Rays Lajolla USA VoL p.soi. 5 Shea M A & Smart D f. Proceedings of 21st International Conference on Cosmic Rays Adelaide Australi a Vo L p Shrivastava P K Proceedings of2 J sl Inlerr-alienal loi1jrrence on Cosmic Rays AdclaidcAustraliaVoL r.os. 7 Shrivastava P K & Agrawal S P Indian J Radio & Spac" p/. ~s 22( 1993) Sykora J Contrb. Astrom. Obs. Skalnale Dleso (Czechoslovakia) 22 (1992) 55.